When someone makes up a new word or expression to describe something, and that word or expression is just so sublime and succinct that it is quickly adopted by the mainstream or the sub-mainstream, necessitating its inclusion in the Urban Dictionary.

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed